Sir Isaac Newton's theories developed much of the foundation of what we know about force and how it affects motion. We witness these laws every day when we see automobiles speed up and slow down, when we throw a ball, even when we simply hold an object above the ground. All three of Newton's laws can be demonstrated easily in a classroom and make engaging class activities.
The Curious Coin
Newton's First Law of Motion states: "an object at rest tends to stay at rest, and an object in motion tends to stay in motion, unless acted upon by an outside force." Set a cup on a table, and place an index card on top. Place a quarter on the center of the index card. Have one student flick the edge of the index card so that it flies away from the cup. The quarter should fall into the cup. Explain that the quarter had inertia -- force acting upon it -- from gravity. When the card was removed that inertia caused the coin to fall.
This activity will demonstrate how the balance of forces work with Newton's Second Law of Motion: that force and mass affect acceleration. Moisten a small wad of tissue paper and plug the opening of an empty soda bottle. Then, place a plastic cup upside down over the plugged opening. Explain that forces either push or pull, and that when they are balanced against each other -- such as the forces inside the bottle and outside the bottle -- nothing happens. Have one student squeeze the bottle. The cup will lift upward. Explain that when the bottle was squeezed, The force upon it increased the pressure inside the bottle, which became greater than the pressure outside. The forces became unbalanced, and the cup was lifted.
The Magic Can
You'll need a long rubber band, a coffee can that has had its bottom removed so that both ends are open, and two plastic lids. Have students help you poke two holes in each lid. Cut the rubber band and thread each end through the holes in one lid. Put that lid on one end of the can. Inside the can, cross the bands, and tie a screw nut to the bands where they cross. Thread the bands through the second lid and tie them. Secure that lid to the can. Have a student roll the can on the floor: it will roll, stop, and then roll back. Explain Newtons Third Law of Motion: "every action has an equal and opposite reaction." As the can rolls, the band twists and builds stored energy. The coil tightens; the can stops and releases the energy, causing it to roll backward.
The Myth of Centrifugal Force
Pour some popcorn into a small bucket and tie a string to the center of the bucket's handle. Then, hold the end of the string and swing the bucket in a full circle. When no popcorn spills out, explain that many people think centrifugal force pressed the popcorn toward the bottom of the bucket, and held it in place. However, centrifugal force doesn't exist. The popcorn stays in the bucket because it's constantly accelerating outward, but held in place by the bucket. Allow students to swing the bucket. Remind them of how it feels to be on a merry-go-round. If they let go of the rail, they would be thrown off by their own momentum, and not by an outside force.
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